If you’ve ever had an inner-ear infection (labyrinthitis) you’re probably familiar with the initially scary symptom — vertigo — first noticed when you try to stand up or otherwise change the orientation of your head and suddenly the world sets to spinning about you. The way to deal with this, until the symptom goes away, is to move very, very slowly, kind of like a sloth.
Sloths move very slowly. With that in mind, if you were going to predict a kind of animal that could tolerate a lot of variation in the architecture of its inner ear, with some presumably being more conducive to precise balance and some less so, what animal might that be? Take a guess. Squirrels? No, wrong, a squirrel with vertigo would not be a very successful squirrel. If you guessed “sloths,” you’re right! You could have been a great 19th-century naturalist. Sloths would seem to have very relaxed needs in terms of balance, and so could afford that kind of variation in a way squirrels could not.
Jerry Coyne at Why Evolution Is True returns momentarily to blogging on evolution with an interesting post, “Darwin right again: the inner ears of sloths are highly variable,” touting a study in Proceedings of the Royal Society B.
The long and the short of it is that in the Origin of Species Darwin “predicted,” insofar as a feature is not useful to an animal, you may expect to find more variation in it: “An organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection.”
The paper confirms this with regard to three-toed sloths as compared to squirrels and some other creatures. Was this not crushingly obvious even before a team of seven international scientists, from Cambridge, the University of Vienna, and elsewhere, went to work on the problem?
By the end of the post even Coyne is backing away from the significance of the finding. It was only

one small prediction that Darwin made. But even that prediction has already been verified in characters like wisdom teeth and human muscles, so the present study doesn’t really add much to substantiating that idea. What the work does suggest, though, is that it’s less important for sloths than for other animals to maintain their balance, and that’s cool in itself.

One of Jerry Coyne’s readers chortles:

It’s always fun to learn more about how the theory of evolution makes testable predictions. Every time I read a post like this I think of my creationist upbringing, but as time goes by I start to just appreciate learning about the world and not letting creationist retardation come into my thoughts.

Yet applying common sense, and the knowledge that species display variations, would have “suggested” that “cool” insight, rendering the “prediction” trivial, without all the trouble this team of researchers went to. By the way, do watch the cute video to which Dr. Coyne directs readers’ attention for a demonstration of just how slow a sloth is.